The optical system of a bar code scanner comprises apparatus for illuminating the bar code tag and apparatus for detecting the reflected light from the bar code tag. To read a bar code, the operator points the scanner at the bar code tag at a distance from the tag within the scanner's working range. In most optical systems, the working range over which the scanner will operate is determined by the apertures of the lenses and the widths of the bars in the bar code being read. Performance is also limited by mounting tolerances for the lenses and the effects of aberration in the lenses.
One type of scanner uses an optical system that forms an image of the bar code tag on the detector, which produces an electrical signal corresponding to the dark and light pattern of the bars and spaces in the bar code. This type of optical system is referred to as "detector resolved". The operation of this type of scanner is limited by the ability of its detector to resolve the optical image of the bar code into an electronic signal.
The detector may be a single element photodetector, or a multiple element photodetector array that spans the image of the bar code. In a single element system, an optical scanning mechanism, such as a rotating mirror, sweeps the image of the bar code across the detector element to produce the signal. This sweep produces a serial image of a "scan line", a line through the bar code approximately perpendicular to the bars and spaces. In a multiple element system, the detector array is electronically scanned.
FIG. 1 shows a diagram of the conventional optical arrangement for a bar code scanner with a multiple element detector. The lens 11 forms an image of the bar code tag 13 to be read on an array of photodetectors 15. Normally, the object plane 14 (the plane of the tag), the plane of the lens 12, and the image plane 16 (the plane of the detector array) are mutually parallel, and are perpendicular to the optical axis 18 of the lens. The elements 17 of the detector 15 are positioned along a line parallel with the scan line for the bar code tag 13, intersecting the optical axis 18 of the lens.
The working range of this optical system is the range of distance from the lens 11 over which the image of the bar code tag 13 remains sufficiently focused for the detector array 15 to recognize bars from spaces and to measure the widths of the bars and spaces accurately. The working range is a function of the aperture of the lens system (the f/#), lens aberrations, and the resolution of the bar code being read.
An alternative type of optical system for bar code scanners uses a laser as its source of illumination. The laser produces a spot that is scanned across the bar code, while a detector collects the reflected light from a field that covers the bar code area or from a field that follows the scanned spot. This type of optical system is referred to as "emitter resolved". The fundamental limitation of this type of system is the tradeoff between the "waist" of the illuminating laser beam and the distance over which the laser beam can be maintained within the required diameter. Diffraction effects due to the aperture for the laser beam limit this distance. For scanning high resolution bar codes with element widths about 0.0075 inch (0.19 mm.), a beam with about 0.010 to 0.015 inch (0.25 to 0.38 mm.) diameter is commonly used. At this beam diameter, diffraction limits the working range to about 7 inches (18 cm.).
Still other optical systems for bar code scanners use light emitting diodes (LEDs) for illumination. These systems, however, typically have even more limited working ranges. In a detector resolved arrangement, the level of illumination available gives rise to the conflicting constraints on aperture size discussed above. In an emitter resolved arrangement, the practical limit of the working range is about half that of a laser system due to the finite size of the LED source. Typical LED bar code scanners have a working range of about 2 inches.
An ideal bar code scanner would have a large working range, while being capable of reading bar codes with very narrow bars and spaces, using a minimum amount of illumination power. However, the optical requirements for these goals conflict. Maximizing the working range requires a small lens aperture (a large f/#), while maximizing resolution requires a large lens aperture because of diffraction effects. A high f/# lens in the collection path also limits the available signal, conflicting with the goal of minimizing illumination power.
An object of the invention is to provide an optical system for a bar code scanner that allows a large working range, high resolution and low illumination requirements.
The invention is an optical system for a bar code scanner in which the working range is not determined by the lens aperture, but primarily by the dimension and orientation of the detectors. In a preferred embodiment, the desired working range is imaged onto a detector array using a modified Scheimpflug arrangement. The detector array in the first Scheimpflug plane with respect to the lens defines a second Scheimpflug plane that intersects the bar code tag to be read.
So long as the bar code tag intersects the image of the detector array in the second Scheimpflug plane, a portion of the image of the bar code tag will be in focus on at least one of the elements of the detector array. The lens aperture can be as large as desired to maximize resolution and minimize illumination needs, and thus power consumption, without adversely affecting the working range.
The optical system of the invention also allows the scanner's bar code illumination to be directed along the second Scheimpflug plane, which is off the optical axis of the lens. This eliminates a fundamental illumination problem of the conventional optical arrangement, that the illumination and detection paths through the optical elements must coincide for good depth of field. With separate illumination and detection paths, the optical system of the invention does not require beam splitters or mirror/aperture arrangements needed by the conventional systems.
In one alternative embodiment, the long axis of the detector array is positioned perpendicular to the long axis of the image of the bar code tag, and a scanning mechanism scans the bar code image across the detector array to produce a scan line. In a second embodiment, the long axis of the detector array is positioned parallel to the long axis of the image of the bar code tag, and a scanning mechanism scans the detector array across the image of the working range to read the bar code tag. It is also possible to move the detector array itself to scan the image of the working range in either of these embodiments. Another alternative is to omit the scanning mechanism, and require the operator to manually scan across the bar code tag.
Another object of the invention is to provide an optical system for a bar code scanner with no moving parts, low power consumption, and with the illumination source not in the same optical line as the detector.
In another alternative embodiment, a two dimensional detector array is used, eliminating the need for the scanning mechanism. The two dimensional array is imaged over the working range in one dimension and across the width of the bar code tag in the other dimension. This system, with no moving parts, provides for a very rugged bar code scanner.